This invention pertains generally to electrical transmission lines, and particularly to circulators and phase shifters adapted to control microwave signals in such lines.
It is known in the art that magnetic ceramic materials, usually referred to as "ferrites," may be used to form different types of elements that control the phase shift of a microwave signal in an electrical transmission line, such as a waveguide. Thus, it is known that a single antenna may be used for transmitting and receiving by: (a) connecting a transmitter through a first transmission line to the H-plane port of a first folded magic-T hybrid junction; (b) connecting a receiver through a second transmission line to the E-plane port of such junction; and (c) connecting the antenna, via a second folded magic-T hybrid junction and third and fourth transmission lines, to the parallel ports of the first folded magic-T hybrid junction. Properly designed and actuated ferrites disposed within the third and fourth transmission lines provide nonreciprocal phase shifts to microwave signals to cause such signals to be directed (when transmitting) from the transmitter to the antenna and to be directed (when receiving) from the antenna to the receiver.
Several problems are encountered in making and operating a circulator such as that just described. For example, proper actuation of a ferrite requires precise control of the strength and direction of a magnetic field interacting with the ferrite. Therefore, although permanent magnets may be used to provide the requisite magnetic field for a ferrite, it has been found to be very difficult to control both the strength and direction of such a magnetic field with a permanent magnet.
Another problem encountered in the fabrication of any known type of circulator derives from the fact that it is relatively easy to induce unwanted modes of propagation within a waveguide. To avoid unwanted modes of propagation (which may cause excessively large insertion losses) extreme care must be taken in design and construction.